Most papers coming from the drying portion of the paper-making machine require a post-treatment for improving their surface structure, especially if they are to be used for printing or writing purposes. The surface of the paper is determined by its smoothness and its sheen. As smoothness is designated the evenness of the paper surfaces; the sheen is a measure for the homogeneous optical reflectivity of the paper surface and is increased by compression thereof. The term covers the entire scale from "high-gloss" to "mat".
To increase smoothness, the paper is conducted through smoothing mechanisms with interacting hard cylinders which flatten the paper. Due to the fact that the paper is compressed more at points of greater thickness and less at points of smaller thickness, non-uniform sheen results from the smoothing, and in the case of higher pressures, even dark spots appear in the paper. To eliminate these irregularities, the paper is subsequently conducted through a calender or supercalender, in which hard and soft cylinders cooperate. The soft cylinders densify the surface of the paper. Because they are not completely rigid at their surface like a metal cylinder, they can adapt to the differences of the paper surface caused by the different densities of the smoothed paper, and can exert a uniformly densifying action thereon. In this effect, friction also plays a role. The soft cylinders are compressed somewhat in the rolling gap, whereby a slight deformation bulge is formed, because the soft cylinder adapts itself to the hard cylinder over a short distance. The deformation leading to the bulge is accompanied by dislocations which also have components parallel to the web surface, so that a friction or massage effect takes place which is of great importance for the surface action.
The so-called soft cylinders can consist of so-called paper cylinders which are made by stacking circular paper disks along the cylinder axis and compressing the stack in that direction. These cylinders are very sensitive at their surface. Every defect of the web is impressed into the surface and remains as a deformation therein, so that grinding or extensive washing of the cylinder is necessary to remove them.
In supercalenders, twelve or more cylinders are arranged on top of another; steel cylinders and soft cylinders alternate. To achieve the surface effect proper, only a few "nips" (cylinder gaps) would be sufficient. The further nips are only required to equalize surface defects of the paper which were generated by surface defects of the first cylinders.
The large number of cylinders on top of each other leads to a heavy, not always desired densification of the paper if the cylinders rest on top of each other with their weight and correspondingly high line pressures prevailing in the lower nips. If this is to be avoided, extensive apparatus for the partial equalization of the cylinder weight is necessary.
To this must be added that disturbances in the operation of a supercalender due to tearing of the paper or formation of creases are quite frequent, so that it must be shut down often. During such shutdown periods, however, very large amounts of paper which cannot be processed further, are produced because of the high operating velocities, up to 1000 m/min, of modern paper-making machines, since the paper-making machine as such must continue to run, of course. In order to prevent these losses, it is customary to connect to the paper-making machine two super calenders outside the paper-making machine proper, so that if one of them fails, the production of the paper-making machine can be transferred to the other.
The overall cost for refining the surface of the paper produced is therefore considerable.
It has already been attempted to use as the soft cylinders, instead of the designs known heretofore, the sensitivity of which has caused a major part of the high costs, cylinders with a working cylinder periphery of different materials. In these experiments, it was found that polyurethane has properties which make it outstandingly suitable for improving the surface finish of paper. The polyurethane is applied to a metal cylinder in liquid form and is cross-linked at its surface to form a dense coating firmly contacting the metal and which, for the purposes under consideration here, has a hardness in the range of about 60 to 75 Shore D.
The effects on the paper attainable with such polyurethane cylinders are therefore at least equal, if not superior, to the soft cylinders customary to date in super calenders, with respect to the attainable effect. They even have a smoothing effect. A particularly important additional effect is, however, that the cylinder material has an enormous restoring ability, so that a surface defect which was impressed into the surface of the polyurethane cylinder, for instance, if a hard spot or a doubling of the paper web has run through, is restored already after one revolution of the cylinder, so that nothing is seen any more on the cylinder surface. This equalizes the surface structure substantially. Since no defects are retained in the surface of the cylinder, it is also possible to dispense with quite a number of cylinders of the calender, which only had the purpose of equalizing the defects caused in the paper web by the surface defects of the soft cylinders. If polyurethane cylinders are used, it is therefore possible to limit the number of cylinders to what is necessary to obtain the effect on the paper.
In spite of all these convincing advantages, polyurethane cylinders could be used so far for improving the surface finish of paper, only in isolated cases at low operating speeds and relatively low pressures, because it has not been possible to make the cylinders durable at higher operating speeds and line pressures. It was found again and again that quite suddenly, the treatment effect on the paper web was diminished at one point of the operating width, and damage to the surface of the polyurethane was found.
Various efforts have been undertaken to control these phenomena by varying the layer thickness of the polyurethane and its composition but these endeavors have not been rewarded to date in any manner. For purposes of the kind under consideration here, cylinders with polyurethane coatings could not be used in production machines up to now.
Applicant has made numerous tests to find the cause for the so far uncontrollable malfunctioning of polyurethane coatings for the improvement of the surface finish of paper. The tests were carried out with an arrangement of two cooperating cylinders, of which one was a deflection-controlled roll or so-called floating cylinder with a working cylinder periphery of steel, and the other cylinder was a conventional cylinder with a polyurethane coating.
If constant properties of the paper web to be processed can be assumed, the mechanical stress of the polyurethane follows from the operating speed and the line pressure. It is clear that due to the deformation of the resilient polyurethane, a certain amount of deformation energy is spent which is produced in each zone when it runs through the cylinder gap. The released power depends on the number of traversals, i.e., directly proportionally to the operating speed. The deformation energy also increases with the line pressure, although not proportionally, since the deformation is not twice as large for twice the line pressure but assumes some value determined by the shape of the deformation bulge.
In the tests, the load was first increased until a recognizable decrease of the treatment effect on the paper occurred. When this point was reached, an important finding was made: The temperature of the surface of the polyurethane cylinder was always surprisingly high in these cases. It was found that the treatment effects of the polyurethane occur on the paper only if the surface temperature of the polyurethane is below 50.degree. C.
The test stand was then equipped with a highly sensitive temperature measuring and recording system which was capable by means of an infrared radiation thermometer to pick up and record temperature differences of one degree of the surface of the polyurethane layer. The measuring head with the radiation thermometer was led back and forth continuously in the lengthwise direction along the cylinder during its operation, so that the temperature on the entire cylinder surface could be measured continuously.
This comprehensive temperature monitoring system showed that it happened during the operation of the polyurethane cylinder that the temperature at a given point rose at some time suddenly very rapidly, i.e., within seconds, without any visible external cause, to values at which thermal decomposition of the polyurethane occurs. These seemingly random temperature increases occur locally and not uniformly on the entire cylinder periphery. It is thus possible that the latter is overall in the range below 50.degree. C. which is considered as usable. Nevertheless, a local temperature increase leading to the destruction of the cylinder occurs sooner or later at some point.
The consideration on the origin of these phenomena led to the discovery that the cross-linked polyurethanes of interest for the coating of cylinders convert an extraordinarily high percentage of the deformation energy into heat. Since, in addition, polyurethane is a poor heat conductor, the heat is removed only slowly from a point at which a temperature rise has occurred. Accordingly, the local increase of the volume that occurred at the respective point due to thermal expansion, is at first retained. The point therefore enters the nip again in the next revolution so-to-speak as a bulge. When traversing the nip the next time, the point now receives increased deformation because first, the bulge is formed back to the old shape of the circumference of the cylinder and then, the normal surface deformation in traversing the nip is added. This in turn leads to an increased production of heat at the same point and thus, to still more thermal expansion of the material there. In this manner, the process builds rapidly until the surface temperature at the respective point reaches values which the material can no longer withstand.
The starting point for such local temperature increases can be the most varied of circumstances, which are all equipment-related and can therefore not be avoided.
A main reason, of course, is an irregularity in the paper web, for instance, a thickening of the mass of the paper in the form of a longitudinal strip or a crease in the paper. The local deformation of the polyurethane accompanying this, if it acts on one and the same spot of the cylinder circumference in several revolutions, can already serve as the germ of a temperature increase which can no longer be stopped.
Another source of these phenomena can be irregularities in the material of the cylinder jacket itself. Even with the greatest care in the fabrication and most thorough mixing of the components, it cannot be precluded that under appropriate concentration conditions of the components, for instance, greater cross-linking of the material and therefore, a kind of hard lump is developed at some point which is not visible from the outside but, with the sensitivity of the material, can be the cause of a local temperature increase which rapidly builds up to a destructive extent.
A third group of influence factors is related to the design of the cylinders, especially to their different deflection lines. In general, it is impossible to set two identical cylinders without deflection exactly opposite each other. Usually, the cylinders have different deflection lines, so that there are zones of higher pressure which are adjacent to zones of lower pressure. This is the case particularly if the deflection lines have inversion points. Although a far-reaching match to the deflection line of the matching cylinder can be achieved through the use of deflection-controlled cylinders such as floating cylinders, in which the cylinder is a tube held on a stationary core in bearings at each end, so that the remaining differences no longer matter for the treatment of the paper, the stress of the polyurethane is not sufficiently uniform but is larger in the zones of higher pressure, so that there, temperature increases due to the part converted into heat by the kneading energy results which can already be the cause for initiating the already mentioned buildup of the temperature increase effect and the "running away" of the temperature.
Finally, the triggering element for this phenomenon can also be a temperature increase which is not generated by the deformation of the cylinder jacket but is introduced into the cylinder jacket directly as a temperature increase, be it by different temperature conditions in the support cylinder or in the matching cylinder, or by temperature differences which come about by temperatures in the paper web which are different in the transverse direction or by an only partial coverage of the polyurethane cylinder by the running paper web. Also these temperature differences introduced from the outside into the polyurethane layer lead to non-uniform thermal expansion and to a deviation of the shape of the cylinder from the ideal cylinder form, which is amplified as the cylinder rotates, in the already described manner.
Therefore, there is a total of possible inhomogeneities, the effects or appearance of which are unforeseeable and which can trigger the local running-away of the temperature. Of course, the effect can be avoided by reducing the operating speed and the line pressure. Then, however, operation is possible only under conditions which are of interest at most in isolated cases. Rather, it is the intent of this invention to allow the use of polyurethane cylinders in the heretofore inaccessible ranges above an operating speed of 350 m/min and a line pressure above 80 kg/cm cylinder length, to obtain a pronounced treatment effect at high throughput velocities.
It is therefore an object of the invention to prevent in a pair of cylinders of the type described, the occurrence of local temperature run-away in the mentioned stress ranges with reliability also for extended periods of operation.